Optional Arguments

How do I give optional arguments (i.e. inputs) to a function? For example, in

shiftByFour :: Bool -> Int -> Int
shiftByFour flag x = if flag then x + 4 else x - 4

I want the Bool flag to be optional, and to have a default value.

Answer

In a literal sense, you cannot have an optional argument.

The way to do this is to make a function take a Maybe Bool instead of a Bool. For example:

shiftByFour maybeFlag x = case maybeFlag of 
    Nothing -> supply-some-default-behavior
    Just flag -> if flag then x + 4 else x - 4

Summary

Instead of optionally having an argument, have a non-optional argument which has an optional type.

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